What tests are used to diagnose Antiphospholipid Syndrome (APLA)?

Diagnostic Tests for Antiphospholipid Syndrome (APLA/APS)

The diagnosis of antiphospholipid syndrome requires three specific laboratory tests performed comprehensively: lupus anticoagulant (LAC), β2-glycoprotein I-dependent anticardiolipin antibodies (aCL) IgG/IgM, and anti-β2-glycoprotein I antibodies (aβ2GPI) IgG/IgM, with all three tests mandatory as they identify different antibody populations and triple positivity confers the highest thrombotic risk. 1

Core Laboratory Tests Required

1. Lupus Anticoagulant (LAC)

• LAC testing uses a multi-step functional clotting assay approach consisting of screening, mixing, and confirmation steps 1, 2
• Two parallel phospholipid-dependent clotting assays must be performed simultaneously: dilute Russell's viper venom time (dRVVT) and an LA-sensitive activated partial thromboplastin time (APTT) 1, 2
• Omitting either dRVVT or APTT increases the risk of underdiagnosis in up to 55% of triple aPL-positive samples 1, 3, 2
• Results are reported as positive or negative (not quantitative) 1, 2

2. Anticardiolipin Antibodies (aCL)

• β2GPI-dependent anticardiolipin antibodies of IgG and IgM isotypes must be measured in plasma or serum 1
• Testing performed by solid-phase assays (ELISA or automated systems) 1
• Positivity defined as levels above the 99th percentile of normal controls 1
• The β2GPI-dependence is mandatory to avoid detecting non-pathogenic antibodies associated with infections or drugs 1

3. Anti-β2-Glycoprotein I Antibodies (aβ2GPI)

• Anti-β2GPI antibodies of IgG and IgM isotypes measured in plasma or serum 1
• Detected by solid-phase assays (ELISA or automated systems) 1
• Positivity defined as levels above the 99th percentile of normal controls 1
• This test was added to the Sydney criteria as an additional criterion, not as a substitute for aCL 1

Critical Testing Requirements

Temporal Confirmation

• All positive tests must be confirmed on two or more separate occasions at least 12 weeks apart to exclude transient antibody positivity 1
• This temporal requirement applies to LAC, aCL, and aβ2GPI equally 1, 2

Comprehensive Testing Approach

• All three tests (LAC, aCL, and aβ2GPI) must be performed together as comprehensive aPL testing, not selectively 1
• Each test may identify different autoantibody populations, and no single test has sufficient sensitivity 1, 4
• Triple aPL-positive patients (positive for all three tests) are at highest risk of thrombosis or pregnancy morbidity 1

Risk Stratification Based on Test Results

High-Risk Profile

• Triple positivity (LAC + aCL + aβ2GPI) carries the highest thrombotic risk 3, 2, 5
• Double positivity with concordant isotype (both aCL and aβ2GPI of the same IgG or IgM isotype) significantly increases diagnostic confidence 3, 2, 5

Lower-Risk Profile

• Isolated LAC positivity without ELISA test positivity confers lower thrombotic risk 1, 2
• Single positive IgM antibodies are considered less clinically relevant than IgG 2, 5

Important Testing Caveats

Isotype Considerations

• IgG isotype antibodies have greater clinical significance than IgM for thrombotic events 3, 2, 5
• Both IgG and IgM should be tested for aCL and aβ2GPI, as IgM may be more relevant in obstetric APS 5

Anticoagulation Interference

• LAC testing during anticoagulation therapy may produce erroneous results 1, 2
• For patients on vitamin K antagonists (VKA), ideally test 1-2 weeks after discontinuation with or without bridging to LMWH 2
• For patients on direct oral anticoagulants (DOACs), pretest DOAC removal procedures can be used 2
• Alternative tests like Taipan snake venom time/ecarin time (TSVT/ET) may be used during VKA therapy, though sensitivity is not 100% 1, 2

Result Interpretation

• Laboratory results must be reviewed collaboratively between a clinical pathologist and a skilled clinician 1
• Low-positive results near the threshold value require cautious interpretation due to potential 10% imprecision of solid-phase assays 3, 2
• Results must be interpreted in clinical context with knowledge of the patient's anticoagulation status 3, 2, 5

Tests NOT Currently Recommended

• Other antiphospholipid antibody tests beyond LAC, aCL, and aβ2GPI are not recommended yet for routine diagnostic use 1
• Antiphosphatidylserine-prothrombin (aPS/PT) antibodies and anti-domain I β2GPI antibodies may have research value but are not included in current diagnostic criteria 2, 6
• Thrombin generation assays show promise but are not robust enough for routine testing 1